“Multipathing” refers to the ability of software running on a host computer to access a volume or a logical unit (LUN) of storage of a data storage array through multiple paths. The software may uniquely identify each path to a LUN via a port of the host computer, a port of the data storage array, and the LUN. In the context of SCSI (small computer system interface), this is referred to as the I-T-L nexus, i.e., Initiator port to Target port to LUN.
Typically, if the software running on the host computer is unable to perform an I/O operation to a volume of the array on an initial path, the software retries the I/O operation to the volume on another path and labels the initial path for path testing. If path testing of the initial path fails, the software marks the initial path as a “dead” path until periodic testing determines that the initial path is functional again.
Nevertheless, the host computer and the data storage array are able to remain operational as a whole since the software can continue to access the volume on one or more remaining “alive” paths. PowerPath® which is offered by EMC Corporation of Hopkinton, Mass. is an example of a multipathing software product.